This invention relates to a method of setting heat-sealing conditions of a thermoplastic sheet.
Heat-sealing is now widely utilized for sealing the opening of containers, making bags, and so on. The degree of heat seal ranges from separable bonding to complete sealing so as not to pass vapors and microorganisms. There are various factors which influence heat-sealing, such as material to be heat-sealed, designed heat seal strength, the type and seal of a heat sealer, line speed, and ambient temperature. However, when these factors are fixed, i.e. when heat-sealing is is conducted as to a predetermined sheet in a predetermined factory, the heat-sealing conditions depend on heat-sealing temperature and pressing period.
Heretofore, the heat-sealing temperature and pressing period were set empirically by repeating trial and error using a real machine. That is, by using a lot of real material with labor, a heat-sealing test was repeated with the varying of the heat-sealing temperature and pressing period in a wide range, and the heat sealed samples thus obtained were evaluated by JIS method(s), specially tensile strength (JIS Z 0238) and visual inspection. Through the test period, industrial production must be stopped, and nevertheless, the heat sealing conditions thus designed were not so accurate. As a result, the heat-sealing conditions were set slightly higher which tend to induce thermal damages at the heat sealed part.